My best advice would be the next time he throws a fit, tell him in a calm voice that he needs to stop, do what he's told, or you will not allow him to do ___________________,(Insert absolute favorite thing--even if it means taking something away, to not being allowed to participate in an outing) and then IGNORE him. He's 8. He understands that his behavior is wrong--especially being abusive to his siblings!
My eldest daughter was the same way. I have not allowed her to go to Girl Scouts until her behavior improved, she missed outings with daddy and the other kids, we've made her pick up the toys by herself instead of yelling at the other kids and being bossy, & she's missed family movie night at home and has had to sit in her room reading books, coloring or whatever she can find in there to do.
I know I sound horribly heartless, but with her, it had to be something dramatic and CONSISTENT!!! TIme-outs are useless at this point...especially after 3 hours. You're probably frustrated beyond belief at that point, and in the end, he still get what he wants, because his behavior hasn't improved.
As far as being abusive to the siblings, I would tell him that is NOT acceptable at any point and put him in his room!!! If he comes out, put him back....sit in front of the door if you have to. THere is absolutely no hitting allowed. PERIOD! Be absolutely adamant on this point. Tell him your the mom, and it's your job to keep all your children safe; and if he chooses to be mean to them, he can't be by them. PERIOD!
As far as unfinished homework...even art... talk to his teacher. Tell her that he is having a hard time finishing his homework at home. Not because he CAN"T do it, but because he WON"T. See if she would be willing to keep him at recesses/specials/ or after school until he finishes. Also start a chart w/ his teacher concerning the homework. When he has a week of completed homework, without any meltdowns, go get him a special treat. (A milkshake from McDonald's work for us).
My eldest in soon to be 11, and is a great kid! But, every now and then she'll have a meltdown. Maybe once a month though....and we warn her that if she doesn't behave, things will get taken away and/or she won't be able to participate in whatever we have planned for the weekend. Usually that snaps her right back. If it doesn't...which is rare...we follow through.
We also tell our kids how much we love them EVERYDAY! For the most part we are blessed with fantastic kids. But I really do believe it's because in our house, there's no empty threats, or promises. We give ONE warning that their behavior is not acceptable, and if their attitude doesn't change, we follow through.
Of course there's reasons why they may be acting naughty, the biggest one being tired, but, they still don't get to act naughty.
I should also mention we don't take everything away immediately. Usually it starts with..."if you don't pick up your toys, then you won't be able to watch tv until they are picked up." IF the bad behavior escalates, then they get to go to their room until they're ready to pick up the toys, and if it's out of control, they go to their bedroom, no tv for the rest of the night, and no outing. Once I ask a child to pick up the toys, I don't pick them up. They will pick up the toys or no privledges. When the task is completed, we will talk about what happened and why. So they understand that they need to behave and do what is asked of them. Usually by the time the toys get picked up, they have calmed down and are able to talk. My kids can't communicate when they're angry...so we wait for silence or near silence from them, so we know they're ready to listen, and they know we'll be more willing to listen to what they have to say
Good luck. It's tough. But really just hang in there. You're the mom and dad and YOU get to make the rules. Just be consistant and follow through.
P.S. sorry this is so long! I've just been there and know how frustrating it can be! But, if you can be consistent, and follow though, there should be a remarkable change! Just stay strong!